Coke deposition is generally experienced when hydrocarbon liquids and vapors contact the hot metal surfaces of petroleum processing equipment. Due to the complex makeup of the hydrocarbons upon elevated temperatures and contact with hot metal surfaces, it is not entirely understood what changes occur in the hydrocarbons. It is thought that the hydrocarbons undergo various changes through either chemical reactions and/or decomposition of various unstable components of the hydrocarbon. The undesired products in many instances include coke, polymerized products, deposited impurities and the like. Whatever the undesired product that is formed, reduced economies of the process is the result. If these deposits remain unchecked, heat transfer, throughput and overall productivity are detrimentally effected. Moreover, downtime is likely to be encountered due to the necessity of either replacing and/or cleaning of the affected parts of the processing system.
While the formation and type of undesired products are dependent on the hydrocarbon being processed and the conditions of the processing, it may generally be stated that such products can be produced at temperatures as low as 100.degree. F.; but are much more prone to formation as the temperature of the processing system and the metal surfaces thereof in contact with the hydrocarbon increase. At these temperatures, coke formation is likely to be produced regardless of the type of hydrocarbon being charged. The type of coke formed, i.e., amorphous, filamentous or pyrolytic, may vary somewhat; however, the probability of the formation of such is quite high.
Carbon formation also erodes the metal of the system in two ways. The formation of catalytic coke causes the metal catalyst particle to be dislodged. This results in rapid metal loss and ultimately metal failure. The other erosive effect occurs when carbon particles enter the hydrocarbon stream and act as abrasives on the system's tube walls.
As indicated in U.S. Pat. Nos. 3,531,394 and 4,105,540 which are herein incorporated by reference, coke formation and deposition are common problems in ethylene plants which operate at temperatures where the metal surfaces in contact with the hydrocarbon are sometimes at 1600.degree. F. and above. The problem is prevalent in the cracking furnace coils as well as in the transfer line exchangers (TLEs) where pyrolytic type coke formation and deposition is commonly encountered. Ethylene plants, originally produced simple olefins such as ethylene, propylene, butenes and butadiene from a feed of ethane, propane, butanes and mixtures thereof. Later developments in the area of technology however, have led to the cracking of heavier feedstocks, because of their availability, to produce aromatics and pyrolysis gasoline as well as light olefins. Feed stocks now include light naphtha, heavy naphtha and gas oil. According to the thermal cracking processes utilized in olefin plants, the feedstocks are cracked generally in the presence of steam in tubular pyrolysis furnaces. The feedstock is preheated, diluted with steam and the mixture heated in the pyrolysis furnace to about 1500.degree. F. and above, most often in the range of 1500.degree. F., to 1650.degree. F. The effluent from the furnace is rapidly quenched by direct means or in exchangers which are used to generate high pressure steam at 400 to 800 psig for process use. This rapid quench reduces the loss of olefins by minimizing secondary reactions. The cooled gas then passes to the prefractionator where it is cooled by circulating oil streams to remove the fuel oil fraction. In some designs, the gas leaving the quench exchanger is further cooled with oil before entering the prefractionator. In either case, the heat picked up by the circulating oil stream is used both to generate steam and to heat other process streams. The mixture of gas and steam leaving the prefractionator is further cooled in order to condense the steam and most of the gasoline product in order to provide reflux for the prefractionator. Either a direct water quench or heat exchangers are used for this cooling duty.
After cooling, cracked gas at, or close to atmospheric pressure, is compressed in a multistage compression system to much higher pressures. There are usually four or five stages of compression with interstage cooling and condensate separation between stages. Most plants have hydrocarbon condensate stripping facilities. Condensate from the interstage knockout drum is fed to a stripper where the C.sub.2 and lighter hydrocarbons are separated. The heavier hydrocarbons are fed to the depropanizer.